A system for real-time (on-line) detection of powder spray particles in a plasma beam is known from EP 0 542 542 B1. The light radiation emitted by the plasma beam is focused on one end of an optical fiber. At the other end of the optical fiber, the light radiation is split into two light beams using a dichroic lens, which are each conveyed to a photodetector. The intensity distribution over time is determined for each light beam in the photodetectors. Using a filter upstream from the photodetectors, suitable wavelength ranges may be filtered out of the light radiation and their intensity curve over time may be determined.
The option of using an optical fiber bundle and to convey the received radiation to a CCD camera is also described in EP 0 542 542 B1.
Another system, in which photodetectors are used for determining the intensity distribution over time of a light radiation emitted from a plasma, is known from U.S. Pat. No. 5,986,277.
Only the intensity distribution of the light radiation in the plasma and the speed and the temperature of the particles may thus be determined in this system.
Monitoring the process variables relevant for the spray process is described in DE 101 40 299 A1. The light radiation is spectrometrically examined and analyzed in this system. The plasma composition, the composition of the spray materials, and the gas and material flow are determined in addition to the speed and temperature of the particles.
However, monitoring the particle beam and the coating formation is not possible in the described system.
A known method for monitoring the coating process is the particle flux imaging method (PFI method). PFI diagnostics is an imaging method that has been developed for industrial use. An optical CCD camera records the luminous area of the spray jet between the source of the spray jet and the coating area, the separation of hot areas and colder zones being implemented via transmission-adjusted gray filters. The method is used for monitoring the particle beam as well as the plasma or high-speed flame spray jet. The intensity characteristics of the beams are detected and reproduced with minimum technical complexity via simple ellipse geometries whose parameters react sensitively to changes in the process parameters. In this way, the PFI method allows monitoring and quality control of the entire spray process all the way to coat formation.
However, a disadvantage of a system which operates according to the PFI method is that the PFI method may be used in a controlling manner only before and after termination of a spray process. In addition, high-resolution process control is not possible with the PFI method since the entire PFI system is not movable by design and is configured in such a way that the entire area between source and coating area is monitored. Monitoring of individual sub-areas of the spray jet is not possible.